1. Field of the Invention
The present invention relates to integrated circuits. In particular, the present invention relates to integrated circuits which convert a digital signal to an analog signal.
2. Discussion of the Related Art
Digital-to-analog conversion using current steering cells are disclosed, for example, in U.S. Pat. Nos. 5,406,285 and 5,541,598. For such current cells to operate properly, the output impedance of such current cells must remain high, even under low operating voltage conditions.
In accordance with one aspect of the present invention, a digital-to-analog converter is formed by a number of current steering cells, each biased to steer, in accordance with an input bit, a current of a current source of a predetermined magnitude into an output terminal. In one embodiment of the current invention, the current source is provided which includes (a) a first transistor having a control terminal coupled to receive a first bias voltage, and an input terminal being coupled to receive a reference voltage; (b) a second transistor having an input terminal coupled to the output terminal of the first transistor; and (c) a differential amplifier having one of its differential input terminals coupled to the output terminal of the first transistor, the other one of its input terminals coupled to receive a second bias voltage, and its output terminal coupled to the control terminal of the second transistor. Such a current source has very high output impedance, since the output impedance of the second transistor (a cascode transistor) is amplified by the high gain of the differential amplifier. Further, the second transistor is allowed to operate in a linear region to provide a wide operating range. Because of the high output impedance of this current source, a high noise immunity is achieved with respect to noise in the output terminal of the current source.
The present invention is better understood upon consideration of the detailed description below and the accompanying drawings.